As having especially excellent mechanical characteristics, fiber-reinforced composite materials comprising, as the intermediate bases, prepregs that comprise reinforcing fibers and matrix resins are widely used not only in sports goods but also in the aerospace industry and other various general industries.
Various methods are employed to produce fiber-reinforced composite materials. Above all, a method of using a prepreg which is a sheet-like intermediate base to be prepared by impregnating reinforcing fibers with a matrix resin is popularly used. The method gives shaped articles by laminating a plurality of such prepregs followed by heating the resulting laminate.
Recently, more lightweight sports goods with higher durability are desired. In particular, for ball game goods that shall undergo great instantaneous shock, such as golf club shafts, baseball bats, tennis and badminton rackets and hockey sticks, it is an important theme to improve the impact resistance of the materials for those goods in order to make them lightweight and have improved durability.
At present, (cured) epoxy resins with excellent mechanical and chemical characteristics including high heat resistance, high hardness, high dimension stability and high chemical resistance are essentially used as the matrix resins for fiber-reinforced composite materials (The term "epoxy resin" is generally used to mean both a prepolymer and a cured product to be obtained by mixing a prepolymer with a curing agent and other additives followed by curing the resulting composition. Unless otherwise specifically indicated, the term "epoxy resin" as referred to herein means a prepolymer.) However, since (cured) epoxy resins are defective in that they are brittle or, that is, their toughness is poor, there often occurs a problem in that fiber-reinforced composite materials comprising them have poor impact resistance.
Various attempts have heretofore been made to toughen (cured) epoxy resins to thereby improve the impact resistance of fiber-reinforced composite materials comprising the resins. To toughen (cured) epoxy resins, in general, methods of improving epoxy resins themselves and also curing agents and methods of adding modifiers to epoxy resins have been proposed.
One example of the methods of toughening (cured) epoxy resins by improving epoxy resins themselves and curing agents is to add thereto epoxy resins with flexible skeletons or flexibility-imparting curing agents. According to this, however, the degree of elastic modulus and the heat resistance of the resulting (cured) epoxy resins are lowered. Contrary to this, another example is to introduce rigid skeletons into epoxy resins and curing agents to lower the crosslinking degree of the cured resins while controlling the toughness, the elastic modulus and the heat resistance of the cured resins. For instance, in ACS Polym. Preprints, Vol. 29, No. 1 (1988), it is reported that the (cured) epoxy resins as obtained from fluorene type epoxy resins and curing agents can have improved toughness while preventing the lowering of their glass transition temperature. However, the proposed improvement requires extremely expensive epoxy resins and curing agents and shall be applied to only limited use.
One example of the methods of adding modifiers to epoxy resins is to add rubber components or thermoplastic resins to epoxy resin compositions to obtain cured resins with high toughness. (The term "rubber" as used herein includes all elastomers except thermoplastic elastomers.)
For example, Japanese Patent Publication Nos. 61-29613 and 62-34251 have proposed the addition of rubber components, in which carboxyl-terminated butadiene-acrylonitrile copolymer rubbers (CTBN) or nitrile rubbers are added to epoxy resins to modify the resins. Some of the proposed techniques have already been put to practical use. In the proposed methods, however, the rubber components added are once dissolved in epoxy resins and thereafter subjected to phase-separation during curing the resins. Therefore, the method are problematic in that it could not always produce the intended modifying results due to the possible change in the morphology of the cured products that depends on the types of the epoxy resins used and the curing conditions employed and that the rubber component added partly dissolves in the cured epoxy resin phase thereby often lowering the elastic modulus and the glass transition temperature of the (cured) epoxy resins.
To solve the problem in the morphology change that occurs in the toughening of (cured) epoxy resins by means of the addition of rubber components thereto, one method of dispersing rubber particles in epoxy resins is known. For example, Japanese Laid-Open Patent Nos. 58-83014 and 59-138254 have disclosed methods of polymerizing monomers such as acrylates and functional group-containing monomers capable of reacting with epoxy resins, such as acrylic acid, in epoxy resins to obtain epoxy resin compositions containing rubber particles formed and dispersed in the epoxy resins, which are to increase the shear strength of the adhesives. In these methods, however, the dissolution of a part of the rubber component in the cured epoxy resin phase is still inevitable and, in fact, these methods could not ensure sufficient heat resistance for the cured resin products.
In the method of adding thermoplastic resins, if thermoplastic resins having a high glass transition temperature are added, it may be possible to obtain cured products which are tough and which maintain their intrinsic heat resistance. However, in order to attain the intended object to obtain highly-toughened products, large amounts of such thermoplastic resins must be added, resulting in the increase in the viscosity of the reaction system and in the difficulty in the handling of the system. Thus, the method has such problems.